The density of surface mounting on electronic circuit boards is continuing to increase as a result of miniaturization of electronic componentsand the narrowing pitch of electrodes. Typically, higher density mounting is accelerating due to development of narrower pitch in 1005 type chips (standard) which have 1.0 mm length and 0.5 mm width, as shown in FIG. 14, and QFP (Quad Flat Package) at 0.3 mm pitch of leads. Higher performance and higher reliability of solder material used for mounting are thus required.
A problem has arisen, from the environmental aspect, in the use of solder material made of alloy of tin and lead (Sn--Pb) because of its lead content. Specifically, when electronic equipment using solder material containing lead are disposed of and exposed to acid rain, toxic lead elutes in a large quantities, causing environmental problems. Accordingly, development of new solder material which does not contain lead (lead-free solder) is required to replace conventional lead-containing solder.
An example of conventional solder is explained next.
FIG. 15 shows a brief sectional view of solder powder of the prior art. Solder powder 21 is mixed with flux to make solder paste which is used in surface mounting technology. The metal composition of the solder paste is a eutectic alloy of tin (Sn) and lead (Pb). The eutectic point of 63Sn--37Pb (composition ratio is percentage by weight), for example, is 183.degree. C.
An example of an electrode, an electronic component of the prior art is explained next. FIG. 14 shows a surface mounting electronic component of the prior art. As shown in FIG. 14, the surface of an electrode 22 of the electronic component is plated with tin (Sn) or solder (90Sn--10Pb).
The lands on an electronic circuit board of the prior art are copper (Cu) which are either non-plated, or surface plated with gold (Au) or solder (90Sn--10Pb).
The lands of the electronic circuit board and electrodes or lead frames of electronic components are bonded by soldering.
The above conventional structure, however, may cause defective solder wettability; an insufficiently diffused layer between the copper (Cu) land and the solder, or between the electrode or lead frame of the electronic component and the solder; and insufficient bonding strength due to creation of brittle intermetalic compounds of copper/tin alloy between the lands and the solder.
Solder wettability here means spreadability of solder when it is melted.
In addition, the prior art has insufficient reliability of soldered portions due to shorting of electronic circuits caused by whiskering of tin (Sn) and cracking caused by the different linear expansion coefficients of copper (Cu) land and solder.
Furthermore, higher density of electronic circuits has steadily led to miniaturization of electronic components, causing partial detachment of chips due to sudden melting of solder at the eutectic point and insufficient wetting.
Partial detachment of chips here means a rise in a part of the electronic component 3 on soldering after it is mounted on an electronic circuit board 5, as shown in FIG. 5. This inappropriate mounting of electronic components causes defective conductivity. In particular, partial chip detachment occurs often with small electronic components such as the 1005 chips mentioned above. This has led to difficulties in making electronic circuits with further higher density.